Photochemical hole burning media

ABSTRACT

A photochemical hole burning medium is composed of a material in which a rare earth complex and a reducing agent is dispersed in a solid matrix. The rare earth complex may be at least one complex selected from the group consisting of a europium (III) crown ether complex, a europium (III) polyether complex, and a europium (III) cryptand complex.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to optical memories for wavelengthmultiple-type high density recording, and more particularly theinvention relates to optochemical hole burning media.

(2) Related Art Statement

Optical recording media in which recorded information can be rewrittento another are broadly classified into the heat mode type and the photonmode type according to the operating principles. In the former,different states: (recorded state/erased state) which are opticallydiscernible from each other are reversely changed by utilizing heatingand cooling of the medium with irradiation of laser beam.Magneto-optical media, phase transition media, organic media, etc.belong to this type. In the photon mode type, an intrinsic energy of alight determined by its wavelength is directly used to cause reversibleoptical changes. Photochromic media and optochemical hole burning (PBB)media belong to this type.

The Persistent Spectral Hole Burning (PSHB) is the phenomenon that whenlaser beam is irradiated upon a solid in which molecules or ions havingoptical absorption ability, a hole persistently appears in the spectrumat a wavelength equal to that of the irradiated beam. The hole burningis an effective measure as a high resolution spectroscopy for thesolids, and is expected to be applied as a wavelength-multiple type highdensity optical memory in case that the width (uniform width) of thehole of the hole is smaller than that (non-uniform width) of theabsorption spectrum. That is, when the hole burning is effected whilethe wavelength of the irradiating laser, a plurality of holesindependent of one another can be formed in a single spot. If bids of 1and 0 are made correspondent to the presence and absence of such a hole,the wavelength multiple recording is feasible, so that optical memoriesat a super high density can be realized. As a material for such anoptical memory, materials into which rare earth ions are introduced areknown.

However, the media that are at a practical level or a near practicallevel are of the heat mode type. In any of the optically recording mediaof the heat mode type, recording is effected by using asingle-wavelength light, which poses a limit upon the recordingcapacity.

On the other hand, the photon mode type is a level of searchingfundamental materials. Among the photon mode type optical media, theoptochemical hole burning media have the merit that the recordingcapacity can be greatly increased by overwriting information data at onelocation at different wavelengths. However, the optochemical holeburning media are still at a level of searching fundamental materials,including the above-mentioned rare earth ion-introduced materials, andmaterials considered preferable for the optochemical hole burning mediaare still at a study level. Therefore, materials which can be used forthe optochemical hole burning media have been desired to be developed.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provideoptochemical hole burning media which can greatly increase the recordingcapacity.

In order to accomplish the above object, the present inventor repeatedlymade strenuous studies on materials in which various complexes weredispersed in a SiO₂ matrix, and consequently be discovered materialswhich can hold holes even at room temperature.

The photochemical hole burning medium according to the present inventioncomprises a material in which a rare earth complex and a reducing agentare dispersed in a solid matrix.

The following are preferred embodiments of the photochemical holeburning medium according to the present invention.

-   (1) The rare earth complex is at least one complex selected from the    group consisting of europium (III) crown ether complexes,    europium (III) polyester complexes, and europium (III) cryptand    complexes.-   (2) The reducing agent is an electron-donating composite compound.    In this preferred embodiment of the optochemical hole burning medium    according to the present invention, the rare earth complex    contributing to the formation of the hole and the reducing organic    molecules contributing to the stabilization of the hole are held in    the form of an electron-donating composite compound in a uniformly    dispersed state.-   (3) The electron-donating composite compound is a silane compound or    a disilazane compound.-   (4) The silane compound is a hexaalkyl disilazane represented by    hexamethyl disilane, and the disilazane compound is a hexaalkyl    disilazane represented by hexamethyldisilazane.-   (5) The electron-donating composite compound is an organic tin    compound.-   (6) The organic tin compound is a compound represented by RSnSnR in    which R is an alkyl group or an aryl group.-   (7) The solid matrix is at least one glass-forming compound selected    from the group consisting of silica, germanium oxide, boron oxide,    phosphorus pentaoxide and tellurium oxide.-   (8) At least one compound selected from the group consisting of    Al₂O₃, Ga₂O₃, In₂O₃, TiO₂, ZrO₂, Nb₂O₅ and Ta₂O₅ is contained in    said solid matrix.

These and other objects, features and advantages of the invention willbe appreciated upon reading of the following description of theinvention when taken in conjunction with the attached drawings, with theunderstanding that any modifications, variations and changes could beeasily made by the skilled person in the art to which the inventionpertains.

As a further preferable embodiment of the photochemical hole burningmedium according to the present invention, the reducing agent has anoxidation/reduction potential of not more than 1 V.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is made to theattached drawings, wherein:

FIGS. 1( a) to 1(c) are graphs showing excitation spectra of SiO₂—ZrO₂:[Eu(15C5)]³⁺ before and after irradiation with laser beams.

FIGS. 2( a) to 2(c) are graphs showing heat cycle characteristics ofSiO₂—ZrO₂:[Eu(15C5)]³⁺ in which FIGS. 2( a), 2(b) and 2(c) correspond toSiO₂:ZrO₂ =7:3, SiO₂:ZrO₂=5:5, and SiO₂:ZrO₂=3:7, respectively.

FIGS. 3( a) and 3(b) are graphs showing fluorescent spectra ofSiO₂:[Eu(15C5)]³⁺ before and after laser beam irradiation, respectively.

FIGS. 4( a) and 4(b) are graphs showing spectra of the optochemical holeburning media according to an embodiment of the present invention beforeand after laser beam irradiation, respectively, in which FIGS. 4( a) and4(b) correspond to SiO₂:Eu(15C5)³⁺Me₃SiSiMe₃ and SiO₂:Eu³⁺Me₃SiSiMe₃.

FIG. 5 is a graph showing spectra of the optochemical hole burningmedium of SiO₂:Eu(15C5)³⁺Me₃SnSnMe₃ according to another embodiment ofthe present invention before and after laser beam irradiation.

FIGS. 6( a) to 6(c) are graphs showing heat cycle characteristics ofoptochemical hole burning media a further embodiment according to thepresent invention in which FIG. 6( a) shows the heat cyclecharacteristic of SiO₂:Eu(15C5)³⁺Me₃SiSiMe₃, and FIGS. 6( b) and 6(c)show the heat cycle characteristics of SiO₂:Eu(15C5)³⁺Me₃SnSnMe₃ andSiO₂:Eu(15C5)³⁺, respectively.

FIGS. 7( a) and 7(b) are graphs showing heat cycle characteristics ofoptochemical hole burning media with SiO₂:Eu(15C5)³⁺Me₃SiSiMe₃ of otherembodiment according to the present invention in which FIG. 7( a) showsthe heat cycle characteristics of in the use of 3 mol % Me₃SiSiMe₃ forSiO₂:Eu(15C5)³⁺Me₃SiSiMe₃, and FIGS. 7( b) the heat cycle characteristicin the use of 6% Me₃SiSiMe₃ for SiO₂:Eu(15C5)³⁺Me₃SnSnMe₃.

FIGS. 8( a) to 8(d) are graphs showing excitation spectra ofSiO₂:M_(x)O_(y) (Si:M=7:3):[Eu(15C5)]³⁺ (Eu³⁺:15C5=1:3) at 77K beforeand after laser irradiation.

FIGS. 9( a) and 9(b) are graphs showing heat cycle characteristics ofhole burning media of the SiO₂:M_(x)O_(y):[Eu(15C5)]³⁺ media irradiatedat 77K.

FIGS. 10( a) and 10(b) are graphs showing heat cycle characteristics ofhole burning media of SiO₂:M_(x)O_(y):[Eu(15C5)]³⁺ at 77K.

FIG. 11 shows excitation spectra of SiO₂:Eu(15C5)³⁺ C₉H₈ (indene) beforeand after irradiation with laser at 77 K and a differential spectrumtherebetween.

DETAILED DESCRIPTION OF THE INVENTION

The photochemical hole burning medium according to the present inventioncomprises a material in which a rare earth complex and a reducing agentdispersed in a solid matrix. That is, the present invention is directedto the optochemical hole burning medium using the material exhibitingthe optochemical hole burning phenomenon.

In the present invention, the term “solid matrix” means host moleculesof the optochemical hole burning medium, and is not particularlylimited. For example, as the solid matrix, at least one glass-formingcompound selected from the group consisting of silica, germanium oxide,boron oxide, phosphorus pentaoxide and tellurium oxide may be recited.Further, at least one compound selected from the group consisting ofAl₂O₃, Ga₂O₃, In₂O₃, ZrO₂, Nb₂O₅ and Ta₂O₅ may be contained in the solidmatrix. From the standpoint of easy productivity with use of a sol-gelmethod, silica may be recited as the solid matrix.

As the rare earth complex, at least one complex selected from the groupconsisting of a europium (III) crown ether complex, a europium (III)polyether complex, and a europium (III) cryptand complex may be recited.

In view of the fact that easy reduction from trivalent to a divalentstate, which is considered to be a factor of inducing the optochemicalhole burning effected, the europium (III) crown ether complex ispreferred as the rare earth complex. As large ring compounds representedby the crown ether, large ring compounds having heteroatoms such asoxygen, nitrogen, sulfur, etc., e.g., 12-crown-4, 15-crown-5, 18crown-6,24crown-8, dibenzo-18-crown-6, cryptand[2, 2], cryptand [2, 2, 2], etc.may be recited. In the present invention, such large ring compounds maybe recited.

From the standpoint of easy complex formation of divalent europium ions,15-crown-5 (hereinafter referred to as “15C5”) is preferred as the crownether.

The rare earth metals are not particularly limited, and Eu, Sm, Pr, etc.may be recited. From the easy complex formation of divalent europiumions, Eu may be recited as the rare earth element.

The reducing agent used in the present invention is not particularlylimited so long as it can readily reduce the are earth ions while notcausing a reverse reaction and its absorption does not overlap with thatof a zerophone line of the rare earth ions. Preferably, organicmolecular compounds which exhibit compatibility with the rare earthcomplex may be recited. From the standpoint of easy transportation ofelectrons with the rare earth ions, the reducing agent may be anelectron-donating composite compound. As the electron-donating compositecompound, a silane compound, a disilazane compound or the organic tincompound may be recited.

As the silane compound, at least one a hexaalkyl disilazane representedby hexamethyl disilane may be recited. As the disilazane or ahexaalkyldisilazane represented by hexamethyl disilazane may be recited.From the standpoint of being readily dissolved in a common solvent to beused in the sol-gel reaction, hexamethyl disilane and disilazanecompound may be recited as the silane compound and the disilazanecompound, respectively.

As the electron-donating composite compound, an organic tin compound maybe used. As the organic tin compound, a compound represented by RSnSnRin which R is an alkyl group or an aryl group may be recited. From thestandpoint of being ready dissolved in a common solvent to be used inthe sol-gel reaction, R is preferably a methyl group.

The use amount of the reducing agent varies depending upon rare ions,complex ligands, solid matrixes, etc. as employed, and is notparticularly limited. From the standpoint of maintaining the high holestability, up to 20 mol % of the reducing agent may be used relative tothe entire amount of the metal component constituting the solid matrix.The use amount is preferably 3 to 6 mol % from the standpoint of thetransparency and light transmission of the medium.

According to a further preferable embodiment of the photochemical holeburning medium of the present invention, the reducing agent has anoxidation/reduction potential of not more than 1.5 V (vs. SCE). Thereason is that the oxidation potential of E³⁺/Eu²⁺ is about −0.43 V (vs.NHE), the Eu is converted to an excited state by irradiation with laserbeam, and Eu³⁺ can be reduced to Eu⁺², if the oxidation/reductionpotential is not more than 1.5 V (vs. SCE).

Therefore, any reducing organic molecule having the oxidation/reductionpotential of not more than 1.5 V (vs. SCE) as the reducing agent cantheoretically reduce Eu³⁺ to Eu²⁺ and can exhibit the hole burningeffect.

Even other organic molecules having an oxidation/reduction potential ofmore than 1.5 V (vs. SCE) can cause hole in relation to other rear earthcomplex. In such a case, the organic molecules having theoxidation/reduction potential of more than 1.5 V (vs. SCE) can be used.

Next, the method for producing the optochemical hole burning mediumaccording to the present invention will be explained. The optochemicalhole burning medium according to the present invention can be producedby using the ordinary sol-gel method, for example. The sol-gel method isgenerally a method in which a gel is obtained by dewatering ahydroxide-containing sol, and an inorganic oxide or the like having agiven shape or in the form of a thin or thick film on a substrate isprepared by heating and drying the gel.

EXAMPLES

The present invention will be explained in more detail with reference tospecific Examples, but the invention is never intended to be interpretedas being limited to these Examples.

Example 1

An optochemical hole burning medium using a solid matrix in which SiO₂was added to ZrO₂ was prepared by the sol-gel method. The preparingprocedure was as follows. A few or several drops of hydrochloric acidwere added as a catalyst into a solution of Si(OC₂H₅)₄:H₂O:C₂H₅OH=1:1:5(molar ratio), which was refluxed for one hour. Then, a metalalkoxide:Zr(OC₂H₅)₄ was added to the resulting solution such thatSi:Zr=7:3, 5:5 or 3:7, followed by one hour refluxing.EuCl₃:H₂O:C₂H₅OH=0.03:4:4:0.03 was added to the resultant, which wassubjected to drying at 50° C. for 2 weeks or 90° C. for 2 days. Thereby,(SiO₂—ZrO₂):[Eu(15C5)]³⁺ was obtained.

After the resulting sample was cooled by using a cryostat, a hole wasformed through being irradiated with laser beam of rhodamine 6G colarantat 100 mW/mm² for 10 minutes. The stability of the hole was evaluatedbased on temperature cycles that the sample having a hole formed at 77Kwas heated to a given temperature, held at this temperature for about 1minutes and cooled again to 77K.

More specifically, the hole was formed by irradiating laser beam at 77Kupon each of samples in which 3 mol % of EuCl₃ and 9mol % of15-crown-5(15C5) were incorporated into a ceramic material formed bymixing SiO₂ with ZrO₂ at a given ratio.

FIGS. 1( a), 1(b) and 1(c) show excitation spectra of ⁷Fo-⁵Do before andafter the laser irradiation upon these samples.

As a result, it was seen that as the content of ZrO₂ in the solid matrixincreased, the non-uniform width was enlarged. Thus, it is consideredthat the local structure near Eu³⁺ ions in the matrix become non-uniformdue to the incorporation of ZrO₂. However, the depth of the hole formeddecreased with increase in the incorporated amount of ZrO₂. Further, ananti-hole was seen in the case of SiO₂:ZrO₂=5:5. This is interpretedsuch that the formation of a complex between Eu³⁺ ions and 15C5 wasinterrupted by the formation of a firm network.

FIGS. 2( a), 2(b) and 2(c) show heat cycle characteristics of(SiO₂—ZrO₂): Eu(15C5)³⁺ each having a hole formed at 77K. FIG. 2( a)corresponds to SiO₂: ZrO₂=7:3, FIG. 2( b) to SiO₂:ZrO₂=5:5, and FIG. 2(c) to SiO₂:ZrO₂=3:7.

When the ingredients constituting the matrix were SiO₂:ZrO₂=7:3, thehole could be maintained up to 300K. When the ingredients constitutingthe matrix were SiO₂:ZrO₂=5:5, the hole could be maintained up to 150K.When the ingredients constituting the matrix were SiO₂:ZrO₂=5:5, thehole could be maintained up to 100K. This revealed that if the ZrO₂ isadded at a high concentration, the hole-forming efficiency decreases andthe hole cannot be maintained at high temperatures, although thenon-uniform width increases.

Example 2

Next, in order to clarify a cause for the high hole-maintainingtemperatures of the above-mentioned composite glasses, R6G laser beamsat an intensity of 300 mWmm⁻² and a wavelength of 579.6 mm wereirradiated upon SiO₂:Eu(15C5)³⁺ (EuCl₃=3 mol %, 15C5=9 mol %) at roomtemperature for 2 hours, and fluorescent spectra were examined beforeand after the irradiation. Results of the fluorescent spectra are shownin FIGS. 3( a) and 3(b). As a result, it was clarified in thelaser-irradiated samples that the intensity of light emission at 570–720nm based on Eu⁺³ ions decreased, whereas fluorescent peak based on Eu⁺²ions newly appeared at around 420 nm.

From the above, it was suggested that the optical reduction from Eu³⁺ions to Eu²⁺ ions was caused as the PSHB mechanism by the laserirradiation.

Example 3

From the results stated in Example 2, it was clarified that thereduction from Eu³⁺ to Eu²⁺ can exhibit excellent hole-maintainingcharacteristic.

Thus, various reducing agents were dispersed in trial into solidmatrixes together with rare earth complexes.

First, tests were performed with a silane compound being used as areducing agent. More specifically, SiO₂:Eu(15C5)³⁺, Me₃SiSiMe₃ wasprepared. The preparing procedure was as follows. A few or several dropsof hydrochloric acid were added as a catalyst into a solution ofSi(OC₂H₅)₄:H₂O:C₂H₅OH=1:1:5 (molar ratio), which was refluxed for onehour. Then, EuCl₃:H₂O:C₂H₅OH:15C5:Me₃SiSiMe₃=0.03:4:4:0.03:0.06 (molarratio) were added to the resulting solution, which was subjected todrying at 50° C. for one week or at 90° C. for 2 days. Thereby,SiO₂:Eu(15C5)³⁺, Me₃SiSiMe₃ was obtained. Loaded compositions fortypical glass materials are shown in Table 1.

TABLE 1 Loaded composition for the typical glass materials TEOS (1:1:5)reflux liquid: EuCl₃: H₂O: C₂H₅OH: 15-crown-5: Me₃SiSiMe₃ (molar ratio)1 0.03 4 4 0.03 0.06 1 0.03 4 4 0 0.06 1 0.03 4 4 0.03 0.03 1 0.03 4 4 00.03

Samples to which neither Me₃SiSiMe₃ nor the crown ether was added wereprepared in the same manner.

In the same manner as mentioned above, SiO₂:Eu(15C5)³⁺, Me₃SnSnMe₃ wasobtained.

With respect to the hole burning characteristic, a holes was formed byusing rhodamine 6G colarant laser. Heat cycle tests were effected suchthat after the hole was formed at 77K, then the temperature wassuccessively raised to 100K, 150K, 200K, 250K and 300 K, the temperatureof 300K was maintained for about 1 minute and returned to 77K again, andan excitation spectrum was measured.

FIGS. 4( a) and 4(b) show excitation spectra and differential spectra ofSiO₂:Eu(15C5)³⁺, Me₃SiSiMe₃ and SiO₂:Eu(15C5)³⁺, Me₃SnSnMe₃ before andafter the laser irradiation at 77K, respectively.

An excitation spectrum corresponding to ⁷Fo-5Do transition of Eu³⁺ ionsin a wavelength range of 579 to 581 mm was observed in the samples notirradiated. When the rhodamine 6G coolants laser was irradiated uponthese samples at a rate of 100 mW/mm² for 600 seconds, a half-valuewidth of 0.125 in was observed as shown in FIGS. 4( a) and 4(b).

In the sample with no crown ether added, no hole was formed, althoughthe intensity of the light emission over the entire spectrum merelydecreased through being irradiated with the laser. This revealed that ascompared with the Eu³⁺ ions alone, the Eu³⁺ ions forming a complex withthe crown ether more readily receive electrons from in the matrixMe₃SiSiMe₃ when in the erected state, so that they can more effectivelyform the hole.

Spectra were observed with respect to SiO₂:Eu(15C5)³⁺ and Me₃SnSnMe₃.FIG. 5 shows an excitation spectrum and a differential spectrum ofSiO₂:Eu(15C5)³⁺, Me₃SnSMe₃ at 77K before and after the laserirradiation.

As a result, the formation of hole was confirmed with respect toMe₃SnSnMe₃ to which the crown ether was incorporated (half-value width0.141 nm). With respect to the sample containing no crown ether no holewas formed. Therefore, it was clarified that as compared with the Eu³⁺ions alone, the Eu³⁺ ions forming a complex with the crown ether morereadily receive electrons from Me₃SnSnMe₃ in the matrix when in theerected state, so that they can more effectively form the holes.

Example 4

Next, the heat cycle characteristic was examined with respect to anoptochemical hole burning medium according to one embodiment of thepresent invention.

First, SiO₂:Eu(15C5)³⁺Me₃SiSiMe₃, SiO₂:Eu(15C5)³⁺Me₃SnSnMe₃ andSiO₂:Eu(15C5)³⁺ were prepared. SiO₂:Eu(15C5)³⁺Me₃SnSnMe₃ was prepared inthe same manner as in Example 3 except that Me₃SnSnMe₃ was used insteadof Me₃SiSiMe₃. Further, SiO₂:Eu(15C5)³⁺ was prepared in the same manneras in Example 3 except that no electron-donating composite compound wasused.

With respect to these samples, the heat cycle characteristic wasexamined. That is, the heat cycle characteristic of a hole formed at 77Kin each of the SiO₂, Eu(15C5)³⁺Me₃SiSiMe₃ and SiO₂:Eu(15C5)³⁺Me₃SnSnMe₃in a temperature range of 77 to 300K were examined. Results are shown inFIGS. 6( a) to 6(c). FIG. 6( a) shows a heat cycle characteristic ofSiO₂:Eu(15C5)²⁺Me₃SiSiMe₃, and FIGS. 6( b) and 6(c) shows heat cyclecharacteristics of SiO₂:Eu(15C5)³⁺Me₃SnSnMe₃ and SiO₂:Eu(15C5)³,respectively.

In the SiO₂:Eu(15C5)³⁺Me₃SiSiMe₃, the hole was maintained up to 300K. Inthe SiO₂:Eu(15C5)³⁺Me₃SnSnMe₃, the hole was maintained up to 250K,

With increase in temperature, the uniform width increases, whereas thedepth of the holes decreases. When the sample contains Me₃SiSiMe₃, theholes having about 70% of that of the holes at 77K with the half-valuewidth of 0.79 nm was maintained at 300K. When the sample containsMe₃SnSnMe₃, the hole having about 61% of that of the hole at 77K withthe half-value width of 0.474 nm was maintained at 250K. Therefore, itis seen that the medium to which the reducing agent is added hasimproved temperature stability of the hole as compared with the crownether complex alone. This is considered such that when Me₃MMMe₃(M═Si orSn) functioning as the reducing agent is incorporated, the M—M bond iscleaved through the reduction to make a reverse reaction difficult tooccur.

Example 5

Next, the heat cycle characteristic was examined while the concentrationof the reducing agent was varied. Samples were prepared according to themethod described in Example 3. Me₃SiSiMe₃ was used as the reducingagent.

Results on the heat cycle characteristic are shown in FIGS. 7( a) and7(b). FIGS. 7( a) and 7(b) show the heat cycle characteristics ofSiO₂:Eu(15C5)³⁺Me₃SiSiMe₃ in which FIGS. 7( a) and 7(b) correspond touses of 3 mol % and 6 mol % of Me₃SiSiMe₃, respectively.

As obvious from FIGS. 7( a) and 7(b), it is seen that the case using 6mol % of Me₃SiSiMe₃ exhibited higher stability of the hole as comparedwith the case using 3 mol % of Me₃SiSiMe₃. This is considered such thatincrease in Me₃SiSiMe₃ increased the amount of Eu(15C5)³⁺, so that theholes became difficult to return correspondingly.

Example 6

Next, spectra were examined when the solid matrix was modified. Solidmatrixes in which Al₂O₃, TiO₂ or Ta₂O₅ was incorporated into SiO₂ wereused. Samples were prepared similarly according to the method describedin Example 1.

Laser were irradiated upon each of these samples, and their excitationspectra were observed. FIGS. 8( a) to 8(d) show excitation spectra ofSiO₂-MxOy (Si:M=7:3):[Eu(15C5)³⁺]³⁺ (Eu³⁺:15C5=1:3) at 77K before andafter laser irradiation.

In each of the cases, the depth of the hole was not conspicuouslydifferent from that in the case with SiO₂ alone.

With respect to the width of the hole, when Al₂O₃ was introduced intoSiO₂, Al³⁺ bonds to non-crosslinking oxygen to form a network as [AlO₄].Thereby, the local structure near the Eu³⁺ ions is strengthened tonarrow the width of the hole. It is considered that similar effect isproduced in the case of the incorporation of ZrO₂ and Ta₂O₅.

To the contrary, when TiO₂ was incorporated into SiO₂, the hole widthtended to increase as compared with SiO₂. This is considered that [TiO₄]bonds to the crosslinking oxygen rather than non crosslinking oxygen.

Next, the heat cycle characteristic of the above samples was examined.FIGS. 9 and 10 show the heat cycle characteristics of holes formed at77K in SiO₂-MxOy:[Eu(15C5)]³⁺.

In each case, the hole was coded the heat cycles down to roomtemperature. Particularly, the incorporation of ZrO₂ and Ta₂O₅ retaineddeeper holes at room temperature as compared with Al₂O₃.

Any of the solid matrixes used exhibited high stability of the holes athigh temperatures. This is considered such that the local structure nearthe Eu³⁺ ions was strengthened and the lattice vibration was suppressedby the addition of the heavy element.

The hole burning medium according to the present invention has theadvantageous effect that signals can be written therein depending uponthe wavelength of the laser beam irradiated.

Example 7

A photochemical hole burning medium was prepared in the same manner asin Example 1, and tested in the same manner as in Example 3 except thatindene was used as a reducing agent.

FIG. 11 shows excited spectra of SiO₂:Eu(15C5)³⁺, indene before andafter irradiation with laser at 77 K and a differential spectrumtherebetween.

As a result, formation of a hole was confirmed. From this result, it isseen that any reducing agent can well function as the reducing agent andform a stable hole, so long as its oxidation/reduction potential isequal to or lower than that of indene. The oxidation/reductionpotentials of organic molecules are summarized in below Table 2.

TABLE 2 reducing agents E⁰(D⁺/D) N,N,N,N-tetramethyl-p-phenylene diamine0.16 N,N,N,N-tetramethyl benzidine 0.32 1,4-diazabicyclo[2,2,2]octane0.57 hexamethyl ditin 0.68 N,N-dimethylaniline 0.76 hexamethyl disilane0.92 triethylamine 0.96 2-methoxynaphthalene 1.42 1,1-diphenylethylene1.52 indene 1.52

It is seen that the reducing agents shown in Table 2 all have theoxidation/reduction potentials lower to that of indene, and are wellused.

The hole burning medium according to the present invention has theadvantageous effect that it enables the wavelength-multiple type opticalmemory operable at room temperature.

1. A photochemical hole burning medium, comprising a material in which arare earth complex and a reducing agent are dispersed in a solid matrix;wherein the photochemical hole burning medium is used at lowtemperatures; and said rare earth complex is at least one complexselected from the group consisting of a europium (Ill) crown ethercomplex, a europium (III) polyether complex, and a europium (III)cryptand complex, wherein said reducing agent is an organic tin compoundrepresented by RSnSnR in which R is an alkyl group or an aryl group. 2.The photochemical hole burning medium set forth in claim 1, wherein saidsolid matrix is at least one glass-forming compound selected from thegroup consisting of silica, germanium oxide, boron oxide, phosphoruspentaoxide and tellurium oxide.
 3. The photochemical hole burning mediumset forth in claim 2, wherein at least one compound selected from thegroup consisting of Al₂O₃, Ga₂O₃, In₂O₃, TiO₂, ZrO₂, Nb₂O₅ and Ta₂O₅ iscontained in said solid matrix.
 4. The photochemical hole burning mediumset forth in claim 1, wherein the reducing agent has anoxidation/reduction potential of not more than 1 V.